[all-commits] [llvm/llvm-project] 7134d2: [SimplifyLibCalls] Fix memchr misoptimization (#10...

[Sergei Barannikov via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 7134d2e9ac36c833bb9ed90363def74fd84f7d13
      https://github.com/llvm/llvm-project/commit/7134d2e9ac36c833bb9ed90363def74fd84f7d13
  Author: Sergei Barannikov <barannikov88 at gmail.com>
  Date:   2024-08-27 (Tue, 27 Aug 2024)

  Changed paths:
    M llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
    M llvm/test/Transforms/InstCombine/memchr-7.ll

  Log Message:
  -----------
  [SimplifyLibCalls] Fix memchr misoptimization (#106121)

The `ch` argument of memcmp should be truncated to `unsigned char`
before using it in comparisons. This didn't happen on all code paths.
The following program miscompiled at -O1 and higher:

```C++
#include <cstring>
#include <iostream>

char ch = '\x81';

int main() {
    bool found = std::strchr("\x80\x81\x82", ch) != nullptr;
    std::cout << std::boolalpha << found << '\n';
}
```



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